The story of Victor of Aveyron has substantial evidence behind it. There are certainly very few feral children, but they do exist.

https://en.m.wikipedia.org/wiki/Victor_of_Aveyron

That's believable, but it can't teach us much because it's not an intervention study, which obviously you can't do…

> According to Jean-Marc Gaspard Itard, Victor was a normal child at birth[citation needed] but was neglected by his alcoholic parents from an early age.

Hmm. Did they know about fetal alcohol syndrome then? It's suggested he was autistic, which would've been true at birth.

That's true. Unfortunately real cases are few and far between, so they're hard to study in any sort of methodic way. Plus, as time goes on and society becomes more pervasive, these cases will get progressively rarer.

I think FASD is a good guess.